Monday, July 24, 2006

No, not about his mug shot at the endpoint of evolution in the illustration above, but about the growing evidence that evolution is not just real but is actually happening to human beings right now."

"From 1970 to 2000, there was a widespread view that although natural selection is very important, it is relatively rare," said Jonathan Pritchard, a geneticist at the University of Chicago. "That view was driven largely because we did not have data to identify the signals of natural selection... In the last five years or so, there has been a tremendous growth in our understanding of how much selection there is."

That insight has only deepened as scientists have gained the ability to read the entire human genome, the chain of "letters" that spell out humanity's genetic identity.

"Signals of natural selection are incredibly widespread across the human genome," Pritchard said. "Everywhere we look, there appears to be very widespread signals of natural selection in many genes and many processes."

Pritchard helped write a recent paper that identified some of those changes. The paper was published in the public access journal PLoS Biology:

Abstract: The identification of signals of very recent positive selection provides information about the adaptation of modern humans to local conditions. We report here on a genome-wide scan for signals of very recent positive selection in favor of variants that have not yet reached fixation. We describe a new analytical method for scanning single nucleotide polymorphism (SNP) data for signals of recent selection, and apply this to data from the International HapMap Project.

In all three continental groups we find widespread signals of recent positive selection. Most signals are region-specific, though a significant excess are shared across groups. Contrary to some earlier low resolution studies that suggested a paucity of recent selection in sub-Saharan Africans, we find that by some measures our strongest signals of selection are from the Yoruba population.

Finally, since these signals indicate the existence of genetic variants that have substantially different fitnesses, they must indicate loci that are the source of significant phenotypic variation. Though the relevant phenotypes are generally not known, such loci should be of particular interest in mapping studies of complex traits. For this purpose we have developed a set of SNPs that can be used to tag the strongest (approx) 250 signals of recent selection in each population.

Washington Post science writer Shankar Vedantam (author of And the Evolutionary Beat Goes On . . .) was online at 3 p.m. ET on Monday, July 24 to discuss the psychology behind our political biases and answer questions about evidence that shows humans are still evolving.